1. Field of the Invention
The present invention relates to a novel polythiol and a process for producing the same, a sulfur-containing urethane-based resin prepared from the polythiol and a lens made of the resin.
The polythiol of the present invention finds wide applications in, for example, raw materials for synthetic resins, crosslinking agents, epoxy resin curing agents, vulcanizing agents, polymerization regulators, metal complex forming agents and biochemical lubricating oil additives in addition to the above-mentioned raw material for the sulfur-containing urethane-based resin.
2. Description of the Prior Art
There have hitherto been used, for example, polythiols formed by the esterification of polyols such as pentaerythritol and trimethylolpropane with mercaptocarboxylic acids such as mercaptopropionic acid and thioglycolic acid for the aforementioned applications. Plastic lenses are lighter in weight, less fragile and easier to dye than inorganic lenses, and hence have been increasingly popular for use as optical elements such as eyeglass lenses and camera lenses.
Resins formed by subjecting diethylene glycol bis(allylcarbonate) (hereinafter referred to as DAC) to radical polymerization have widely been used to date for optical elements.
The resins have a variety of features such as excellent impact resistance, light weight, prominent dye-affinity and superb machinability including cutting ability and polishing ability.
However, lenses made from DAC have a smaller refractive index (n.sub.D =1.50) than inorganic lenses (n.sub.D =1.52). In order to obtain equivalent optical properties to glass lenses, it is necessary to increase the center thickness, peripheral thickness and curvature of the lens and hence the lens as a whole becomes unavoidably thick. Therefore, lens-making resins with higher refractive index are desired. As one of the lens-making resins giving higher refractive index, there has been known a polyurethane-based resin for making plastic lenses, which is obtained by reacting an isocyanate compound with a hydroxyl compound such as diethylene glycol (Japanese Patent Laid-Open No. 136601/1982), with a halogen-containing hydroxyl compound such as tetrabromobisphenol-A (Japanese Patent Laid-Open No. 164615/1983), and with a diphenyl-sulfide-linkage-containing hydroxyl compound (Japanese Patent Laid-Open No. 194401/1985).
Although lenses made of these prior art resins have improved refractive indices over lenses made from DAC, their refractive indices are still insufficient. Moreover, these resins have such defects as poor weather-ability and impact resistance or large specific gravity due to the presence of a number of halogen atoms or aromatic rings in the molecule for the purpose of improving their refractive indices.
The present inventors have found earlier that an S-alkyl thiocarbamate resin obtained by reacting pentaerythritol tetrakis(2-mercaptoacetate) (hereinafter referred to as PETG) with xylylene diisocyanate (hereinafter referred to as XDi) has a high refractive index, is colorless and transparent and has excellent mechanical properties and machinability (Japanese Patent Laid-Open No. 199016/1985).
However, even the S-alkyl thiocarbamate resin had such defects as insufficient refractive index, thick peripheral thickness and reduced fashonability when the lens was used for strong sizing of visual acuity.
In consequence, the present inventors have made further examinations to find out a novel compound, 1,2-bis(2-mercaptoethylthio)-3-propanethiol (hereinafter referred to as GST), and found that resins prepared from GST have higher refractive indices than the resins from PETG and are colorless, transparent and excellent in dye-affinity so that they are distinguished as plastic lens materials (Japanese Patent Laid-Open No. 270859/1990).
However, the resin formed by reacting GST with XDi, which is most generally used as a raw material of a plastic lens, has a glass transition temperature of 98.degree. C., and therefore the lens made of the resin is deformed at a common dyeing temperature for plastic lenses of 90 to 95.degree. C., which is close to the critical heat-resistant temperature of the resin. Hence, the lens needs reheating for its restoration, but this is troublesome.
Consequently, the inventors have made further examinations, and found 2-mercaptoethylthio-1,3-propanedithiol (hereinafter referred to as GMT) which gives heat-resistant resins that are not deformed even at the common dyeing temperature (90-95.degree. C.) as well as have equivalent optical properties to resins made from GST (Japanese Patent Laid-Open No. 208950/1993).
However, GMT is a trifunctional thiol and hence, when it is combined with a bifunctional isocyanate to produce a resin, a crosslinked structure of the resin is formed at the final stage of the polymerization so that viscosity increase during the polymerization is slow and resin additives such as a plasticizer usually bleed into the molding from a resin-made gasket used for molding plastic lenses, thus impairing the transparency of the molding. To solve this problem, the polymerization is carried out at a low temperature for a long time and then completed at a high temperature by way of example. This case however gives a prolonged polymerization time and reduced productivity.